Chroma key, or chroma key effect, is a process for mixing two images or frames together, in which a color—or a range of colors—from one image becomes transparent for viewing portions of a different background image instead of an original background associated with the image.
Mixing such an image having transparent regions with a second image creates a composite image. This technique is commonly used to replace a first background with a second background. For example, one or more portions of a first image can be overlaid on a second image to make it appear as though a subject of the first image was originally taken with respect to a background of the second image. In other words, the chroma key effectively keys out or removes all image pixels that are similar to a specified background key color so that the keyed out color, or range of colors, becomes transparent for the entire clip.
One common use of the chroma key process is with weather forecasts. In such applications, a person presenting a weather forecast on television appears to be standing in from of a large weather map. In reality, that is, in the broadcast studio, the weather presenter is standing in front of a large blue or green background. When televising the weather forecast, the blue or green background is keyed out according to conventional techniques and replaced with a weather map or graphical forecast.
Another common use of the chroma key process is in creating special effects in motion pictures. Actors are filmed in front of a greenscreen or bluescreen. (Any color screen can be used, but filmmakers prefer green and blue since these colors are the colors least like skin tone.) During processing of the video images, the background screen is removed, and then replaced with another background such a computer-generated landscape.
Professional, amateur, and hobby filmmakers all can use conventional chroma key techniques to create special effects in video and other types of imaging applications.